1. Field of the Invention
The subject invention concerns a dewatering fabric to be used as a wet press fabric in the press section of a papermaking machine.
2. Description of the Prior Art
A papermaking machine comprises three different sections. In the forming section the stock suspension is fed onto traveling forming fabric or between two such fabrics. The majority of the water is removed from the stock, so that a continuous paper sheet is formed on the fabric. The formed sheet is carried into the press section, where some more water is removed by pressing. Finally, the sheet is dried in the drier section by being pressed against hot cylinders, so that the moisture in the paper sheet is vaporized.
An important part of the papermaking process is the dewatering efficiency in the press section. It is much more economical to remove the water in the press section than to vaporize it in the dryer section. The energy consumption is considerably higher in the dryer section than in the press section.
In the press section of the papermaking machine the formed sheet is pressed to a higher dry content through repeated pressings, usually in roll press nips. The sheet is carried through the press nip together with one or several endless textile fabrics, that are commonly referred to as press fabrics, and may also be referred to as press felts or wet felts.
The press fabric usually includes a soft surface layer next to the paper web, which surface layer is compressed to a rigidity without any air volume. Under the surface layer is usually arranged a base fabric, which is designed to retain most of its void volume, even when a maximum pressure is applied on the press fabric.
The purpose of this design is to enable the fabric to accept an optimal amount of water from the paper web during the compression of the web and the fabric in the press nip, and, after exiting the nip, to retain as much of the removed water as possible for later release in a suitable manner, before the fabric reenters the press nip.
In a currently common type of roller press, the bottom press roller is formed with cavities in the form of suction holes, connected on the inside to a vacuum source, or with lengthwise extending grooves (known as a Venta or grooved roll) or with blind drilled holes. The cavities in such a roll completely or partly replace the base fabric part of the press fabric or supplement the press fabric as a water-absorbing medium, when the paper sheet and fabric are compressed in the press nip. Normally, grooved and blind-drilled press rolls are used at the end of the press section at high linear pressures and high speeds.
When the paper sheet together with one or several press fabrics is carried into the press nip, the water from the paper web is forced into the fabric and then, together with the amount of air stored in the surface layer of the fabric, is forced backwards into the void volume of the base fabric and/or into the void volume of the press roll. Some water is also allowed to flow forwards or backwards in the lengthwise direction inside the fabric. The relationship between these flow directions depends, for example, on the speed of the machine and on the design of the fabric and on its ability to handle the water removed from the web.
Several theories have been proposed to explain what is going on in the paper web and press fabric during the press process itself. The exerted nip pressure is the same for both paper web and press fabric, while the hydrodynamic pressure is considerably higher in the web than in the fabric. This pressure difference provides the driving force for the transportation of the water from the web to the fabric.
The paper web, or sheet, and press fabric probably reach minimum thickness at the same time somewhat after mid nip. The sheet is considered to reach its maximum dry content at the very same moment. After that, the sheet, as well as the fabric, begin to expand. During this expansion, a vacuum is created in the paper sheet and in the surface layer of the press fabric, both of which have been totally compressed when the compression is at a maximum. In response to this vacuum, water flows back from the inside and base layers of the fabric to the surface layer of the fabric and into the sheet to reestablish the pressure balance. This phase provides the driving force of the re-wetting of the paper sheet inside the press nip.
In the press fabric constructions of the prior art, it is common practice to form the fabric with a surface layer facing the paper web considerably denser than the backside structure, and it has not been unusual to use lengthwise extending fibers on the web facing side to decrease flow resistance. High capillary forces, together with the large vacuum in the press fabric structure during the expansion phase, absorb water from an open backside structure toward the surface layer, rapidly decreasing the vacuum in the surface layer. When the vacuum of the sheet thus rises considerably during exit form the press nip and the flow resistance in the contact face of the press fabric against the sheet decrease, high re-wetting and low paper dry content result.
The purpose of the present invention is to create and, above all, to maintain a vacuum pressure which is as high as possible in the surface layer of the press fabric during the expansion phase by counter-acting the water-flow from the interior of the press fabric to the side facing the paper web to inhibit and reduce re-wetting.